1. Field of the Invention
The present invention relates to a coating apparatus and a process for producing an optical film using the same, and more specifically to a coating technique for preventing stepped irregularities when producing an optical film used for liquid crystal display devices.
2. Description of the Related Art
Optical films such as anti-reflection films are used for various image display devices such as liquid crystal display devices (LCDs), plasma display panels (PDPs), electroluminescence displays (ELDs) and cathode ray tubes (CRTs). Anti-reflection films are also used for lenses of glasses and cameras.
For such anti-reflection films, multilayer films prepared by stacking transparent thin films of metal oxide have been generally used so far. The reason for using a plurality of transparent thin films is to prevent light from reflecting in as wide a visible wavelength range as possible. These transparent thin films of metal oxide are formed by a chemical vapor deposition (CVD) method or a physical vapor deposition (PVD) method, and more specifically, a vacuum deposition method or a sputtering method which is one of the physical vapor deposition methods.
However, the chemical vapor deposition (CVD) methods and the physical vapor deposition (PVD) methods are inefficient, and thus instead of them, a coating method in which a coating solution for an anti-reflection film is applied to a support has been employed.
Examples of coating apparatuses for forming a coating film by a coating method include bar coaters, reverse roll coaters, gravure roll coaters and slot die coaters (extrusion coaters). Since slot die coaters are capable of forming a thinner layer at a higher speed compared to other coating apparatuses, they have been frequently used as a coating apparatus for forming an optical film.
In the production of an anti-reflection film by coating of a thin layer using such a slot die coater, the uniformity in film thickness has a great influence on the performance of products, and therefore extremely severe conditions are required. Recently, in particular, there has been a growing demand for a technique of precise coating in a region of 10 μm or less, which is thinner than conventional wet film thickness, to achieve the desired functions.
In other words, the above optical films need to have a strictly accurate coating film thickness and coating film properties and require highly accurate thin film coating technique. For anti-reflection films, in particular, a very small variation in film thickness (stepped irregularities) causes a change in the antireflection rate, and the desired antireflection ability will not be shown.
To achieve highly accurate thin layer coating using a slot die coater, it is necessary to form a stable and uniform coating solution bead between a slot die and a web. The bead becomes unstable because of the vibration of the slot die or peripheral equipment or the variation of pressure reduction value, and in particular, liquid feed pulsation when supplying a coating solution to a slot die is considered to be the major cause.
The liquid feed pulsation directly causes a variation in the discharge rate of a coating solution through a slot die and causes a change in the shape of the bead. As a result, surface defects of thick film and thin film alternately formed in the running direction of a support, which are so-called stepped irregularities, are generated on the coated surface of the support.
The liquid feed pulsation of the coating solution is caused by various factors such as pulsation in a pump which supplies a coating solution, the influence from a defoaming apparatus arranged in a liquid feed line or vibration of a pipe line, and it is very difficult to solve this problem.
Various proposals have been made to solve the problem of the liquid feed pulsation of coating solution (see Japanese Patent Application Laid-Open Nos. 5-31434, 2003-340355, 2004-351291, 2003-10761 and 2002-45761), but the troubles caused by the pulsation of coating solution have not yet been completely solved.
Under such circumstances, the applicant has proposed formation of an orifice in a pocket or at a liquid feed port of the pocket of a slot die as described in Japanese Patent Application Laid-Open No. 2007-245024.